Tunable optical waveguide gratings are employed as dispersive elements in a wide variety of optical devices such as tunable filters, multiplexers and lasers. For example, an interconnection apparatus for optical switching, multiplexing, and demultiplexing is composed of a plurality of closely spaced input waveguides communicating with the input of a star coupler which functions as a splitter. The output of the star coupler communicates with a second star coupler via an optical grating consisting of an array of optical waveguides. Each of the waveguides differ in length with respect to its nearest neighbor by a predetermined fixed amount. The outputs of the second star coupler form the outputs of the switching, multiplexing, and a demultiplexing apparatus. See, for example, U.S. Pat. No. 5,002,350 issued Mar. 26, 1991.
The optical grating located between the two star couplers is essentially composed of an array of curved waveguides of different lengths. The length of each waveguide in the grating differs from the lengths of all the other waveguides in the grating so that, in effect, predetermined and different phase shifts are applied to optical signals propagating into the waveguides of the grating from the star coupler. This phase shift occurs because of the different path lengths over which the signals in the grating must travel to reach the grating output. The output of the waveguides in the grating thus have different phases, which are functions of the lengths of the waveguides.
The phase shifts applied to the optical signal can be altered by providing each waveguide of the optical grating with a suitable electrode controlling the refractive index of the waveguide. For instance, in a semiconductor device, the current applied to the electrode may be used to change the carrier concentration, thus causing a change in the phase of the signal propagating in each waveguide (see, for example, "Guided-Wave Optoelectronics", Ed. by T. Tamir, Springer-Verlag, 1988). An optical waveguide grating having such control electrodes to vary the phase shifts of the signal is referred to as a tunable waveguide grating.
A tunable grating which is composed of a large number of individual waveguides is difficult to implement because it requires a large total electrode area which in turn requires a large total tuning current. Specifically, if N is the number of individual waveguides forming the grating, the total tuning current increases quadratically with increasing N for large N. For example, if the optical path length is to vary be .delta.1 in the k=2 waveguide, the total tuning current required is: ##EQU1## where (k-1).delta.1 is the path length variation that must be produced in the kth waveguide relative to the preceding waveguide and .alpha..delta.1 1 is the current required to produce .delta.1. The constant .alpha. is a function of the material properties of the waveguide and its cross-sectional area, but is independent of electrode length. As seen from the above expression, the required current for large N increases quadratically with N, thus making this technique impractical for tunable gratings having a large number of waveguides. Moreover, the required current is the same whether or not synchronous filter tuning with single-knob current control is employed.
Until now, there has been no convenient way to produce a tunable optical waveguide grating which has a large number of waveguides without requiring a large tuning current.